1. Field of the Invention
The present invention relates generally to methods and devices for producing seamless tubular films having a cellulose base, as well as to cellulose-based seamless tubular films. In particular, the present invention relates to processes of extruding an aqueous cellulose-N-methyl-morpholine-N-oxide (NMMO) spinning solution through an annular die in a spinning bath, whereby the spinning solution is formed into the tubular film, which is drawn laterally, in an air gap between the annular die and the level indicator of the spinning bath, by a supporting burst of excess air pressure in the tubular films.
2. Description of Related Art
The production of cellulosic molded components, especially fibers, films, and tubular films, via a viscose process is widely known in the art, as are problems associated with this process, such as the large number of process stages, and costly measures required for the treatment of waste water and exhaust air. One alternative to the viscose method involves the direct dissolution of cellulose in an organic solvent, and the spinning of such a solution in a spinning bath.
The ability of tertiary amine oxides, under certain conditions, to dissolve cellulose is known for example, from U.S. Pat. No. 2,179,181. The production of solutions using the tertiary amine oxide N-methyl-morpholine-N-oxide (NMMO) and cellulose is described, for example, in U.S. Pat. No. 3,447,939. In U.S. Pat. No. 4,246,221, the production of molded cellulosic components by dissolving cellulose in a mixture of NMMO and water, and spinning such solutions in an aqueous spinning bath is disclosed. Any method of the type described above will hereinafter be referred to as an xe2x80x9camine oxide method.xe2x80x9d All of these documents are incorporated herein by reference.
In WO 93/13670, incorporated herein by reference, the production of a seamless tubular covering for food via the extrusion of a solution of cellulose in NMMO/H2O using a special extrusion die is described. Between the extrusion die and the spinning bath is an air gap. A distinguishing feature of this method is a specially formed hollow mandrel, through which the spinning liquid can also circulate inside the tube. In the air gap, the inside of the extruded tube is almost completely filled by the hollow mandrel and the spinning liquid. The tube is not drawn laterally.
In WO 95/35340, incorporated herein by reference, a method for producing blown cellulose films is described, that employs cellulose that is not dissolved in amine oxides, and not derivatized. The spinning solution is extruded downward through an annular die, through an air gap, in a spinning bath, wherein the tube is filled with a liquid, while at the same time an air gap is established inside the tube by varying the internal bath column. The tube is blown using a propellant or the air that is enclosed inside the tube between the inner bath and the die, whereby an intended orientation of the film may be established, thus allowing increased stability, especially in a crosswise direction, to be obtained.
It is considered particularly disadvantageous relates to the extrusion of a tubular film from an annular die in a spinning bath, wherein no mechanical support of the tubular film is provided inside the spinning bath. This is because the sizing precision of the tubular film cannot be maintained when it is filled with an inner bath solution, which in turn results in irregular variations in the diameter of the tube. This is caused by variations in pressure in the inner air gap that is enclosed between the die and the inner bath level indicator, resulting from variations in the inner bath column caused by exchanges between the tube containing the NMMO and the spinning medium. It is furthermore considered disadvantageous that during the course of the spinning process, the exchanges between the tube containing the NMMO and the spinning medium, result in a constant change in the concentration of NMMO in the inner spinning solution. Since that the spinning conditions are constantly changing, constant film structure and constant film properties are difficult to obtain.
In contrast, in EP-A 0 899 076, incorporated herein by reference, a method and a device are described, whereby a seamless tubular film having a cellulose base can be produced from a solution of cellulose in NMMO/H2O, which does not typically possess the above-named disadvantages. By using spatially separate inlet and outlet pathways for the spinning bath into the inside of the tube, a constant inner bath level and a constant inner bath concentration are established, so that tubular films having a constant size and constant structure can be produced. In order to obtain improved mechanical properties, the tube is drawn laterally in the air gap via the regular introduction of compressed air into the inside of the tube, while a constant internal pressure is continually maintained.
DE-A 196 07 953, incorporated herein by reference, describes the production and use of seamless tubular films having a cellulose base as sausage casings, which are produced by extruding a spinning solution composed of cellulose, NMMO, and water through an annular die and an air gap in a spinning bath.
Methods for producing spinning solutions composed of cellulose, NMMO, and H2O are generally known. In accordance with the state of the art, in the amine oxide method, a spinnable solution is obtained from a suspension composed of cellulose and aqueous NMMO, which in the concentration used and at the applied temperature represents a non-solvent for the cellulose, at increased temperature and shear by removing water under a vacuum until an NMMO concentration is reached that corresponds essentially to the monohydrate concentration of the NMMO, by dissolving the cellulose. In this, the spinning solution may contain an amount of cellulose that is between 5 and 20% by weight, preferably between 7 and 15% by weight, based upon the total weight of the spinning solution. Other naturally occurring and/or hydrophilic synthetic polymers, and polymers that possess both hydrophilic and hydrophobic properties may be contained in the spinning solution. (See, i.e. DE-A 196 07 953 which is incorporated herein by reference).
In order for such cellulosic tubular films to be used as sausage casings, it is desirable and often necessary, that in addition to a precise, constant size, that the sausage casings be easily peelable after use, without their contents (the sausage) becoming damaged. Consistency in size, or a uniform circumference with no kinks, folds, etc., are also often a prerequisite for problem-free filling of the casings, and subsequently for the uniformity of the sausages as final products in terms of their dimensions and surface quality. The ease in peeling the sausage casings can be improved via a special impregnation of the insides of the casings. These so-called xe2x80x9ceasy-peelxe2x80x9d inner preparations can be applied either during the drying process or during filling, via an inner filling die. These methods have already been described, for example in U.S. Pat. No. 3,898,348, EP-A 0 180 207, EP-A 0 635 213, and U.S. Pat. No. 5,358,784, all of which are incorporated herein by reference. An improvement in the ease of peeling either without the xe2x80x9ceasy-peel preparationxe2x80x9d or using smaller quantities of the preparation (and the associated lower production costs) may also be achieved, however, by obtaining the most uniform and smooth inner surface structure possible. As a measurement of the xe2x80x9csmoothnessxe2x80x9d of the inner surface of the tubular film, its degree of roughness may be used, which can be determined using a perthometer.
The perthometer is an electrical measuring device designed to produce profile recordings, measured data displays, and logging of measurements of technical surfaces. A measuring pin uses a vertically movable tip to sense the shape of the surface along the section to be measured. Movements in the pin tip are transmitted to an electronic data converter, which converts them to electronic data that represent the sensed profile. In accordance with DIN-Norm 4766, different characteristic roughness values Ra, Rz, Rmax can be identified. The arithmetic average roughness Ra is the arithmetic average of the absolute values of the profile deviations along a reference section 1. It is equal to the height of a rectangle, the length of which is equal to the reference section 1, and which is equal in area to the sum of the surface area that lies between the roughness profile and the center line. Usually, the Ra value is established within a total section 1m, which is composed of five individual sections 1e lined up in a row, one after the other. The average degree of roughness R2 is the arithmetic average of the individual roughness degrees of five adjacent individual measurement sections. The maximum degree of roughness Rmax is the magnitude of the individual degrees of roughness occurring over the entire measurement section 1m.
From EP-A 0 807 460 and DE-A 197 50 527 (which are incorporated herein by reference in their entireties) it is known that spinning solutions comprising of cellulose and/or modified cellulose, a tertiary amine oxide, and other fluids and additives such as stabilizers, pore builders, etc. can be spun to form cellulosic separating membranes in the form of flat, tubular, or hollow fiber membranes. In this, the structure and thus the separating power of the membrane is influenced to a large degree by the spinning medium and/or by the composition of the spinning medium, by the spinning bath temperature and the concentration of the cellulosic polymer in the spinning solution. Similar statements regarding influences to the structure of blown films produced via the amine oxide process are also published in xe2x80x9cDas Papierxe2x80x9d December 1997, pages 643 through 652, which is incorporated herein by reference.
An object of the present invention is to improve upon prior methods so that a seamless cellulosic tubular film of generally constant size and that is substantially free from defects in terms of undesirable folds, and possessing a uniformly low degree of surface roughness on the inside surface of the tubular film, can be obtained.
The present invention that also possesses provides a method for producing a seamless tubular film based on cellulose comprising extruding an aqueous cellulose-N-methyl-morpholine-N-oxide (NMMO) spinning solution through an annular die in a spinning bath, wherein the spinning solution forms the tubular film. The film is laterally drawn in an air gap between the annular die and the spinning bath by stabilizing air pressure in the tubular film. The level of hydrostatic pressure from an inner spinning solution formed inside the tubular film extends above the spinning bath, the inner spinning solution comprising an aqueous NMMO solution that supports and intensifies said lateral drawing. The tubular film that has been expanded by the lateral drawing process is passes by a turning point in the spinning bath, and directed out of the spinning bath.
The present invention is further directed to devices that are useful in preparing tubular films as well as to tubular films, per se.
In accordance with the invention, objects of the invention can be attained in that the hydrostatic pressure from an inner spinning solution comprising of an aqueous NMMO solution, which is above the level of the spinning bath and is inside the tubular film, supports and intensifies lateral stretching of the film, and further in that the tubular film that is extended by the lateral stretching passes a turning point in the spinning bath after which it is directed out of the spinning bath.
Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the appended claims.